Sensitivity to relative and absolute motion

The threshold of sensitivity to movement could be governed by mechanisms that are sensitive either to change in spatial position, or directly to the movement itself. The use of spatially complex patterns (random-dot patterns) has been suggested to eliminate the former strategy allowing examination of the movement detecting mechanisms in isolation. By means of such a technique, thresholds for directional judgements were determined for patterns which underwent either a simple displacement or a shearing displacement. Thresholds for shearing motion were found to be around one half of those for simple motion, suggesting that relative, rather than absolute, motion governs performance for small displacements. This contrasts with previous experiments which showed that absolute motion governs performance for much larger displacements.